A magnetic valve or a solenoid injector may be used for injecting fuel into a combustion chamber, such as a cylinder. A solenoid injector of said type (also referred to as coil-type injector) has a coil which generates a magnetic field when current flows through the coil, whereby a magnetic force is exerted on an armature such that the armature is displaced in order to effect an opening and closing of a nozzle needle or of a closure element for the purposes of opening and closing the magnetic valve. If the magnetic valve or the solenoid injector exhibits a so-called idle stroke between armature and nozzle needle or between armature and closure element, then a displacement of the armature leads to a displacement also of the closure element or of the nozzle needle not immediately but rather only after the armature has been displaced by the magnitude of the idle stroke.
When a voltage is applied to the coil of the magnetic valve, electromagnetic forces cause the armature to be moved in the direction of a pole piece. By means of a mechanical coupling (for example mechanical contact), after the idle stroke has been overcome, the nozzle needle or the closure element likewise moves and, in the case of corresponding displacement, opens up injection holes for the feed of fuel into the combustion chamber. If current continues to flow through the coil, the armature and nozzle needle or closure element move further until the armature arrives at and abuts against the pole piece. The distance between the abutment of the armature against a driver of the closure element or of the nozzle needle and the abutment of the armature against the pole piece is also referred to as needle stroke or working stroke. To close the valve, the excitation voltage applied to the coil is deactivated, and the coil is short-circuited, such that the magnetic force is dissipated. The short-circuiting of the coil results in a polarity reversal of the voltage owing to the dissipation of the magnetic field stored in the coil. The magnitude of the voltage is limited by means of a diode. Owing to a restoring force which is provided for example by a spring, the nozzle needle or closure element including armature are moved into the closed position. Here, the idle stroke and the needle stroke are passed through in the reverse sequence.
The time of the start of the needle movement during the opening of the magnetic valve is dependent on the magnitude of the idle stroke. The time of the abutment of the needle or of the armature against the pole piece is dependent on the magnitude of the needle stroke or working stroke. The injector-specific time variations of the start of the needle movement (opening) and of the end of the needle movement (closing) may, in the case of identical electrical actuation, result in different injection quantities.
According to the prior art, idle strokes and working strokes are at present mechanically set and measured during the injector assembly process or valve assembly process. It has however been identified that the idle stroke and/or the armature stroke of a magnetic valve may vary over the service life or over the operating duration as a result of running-in processes or wear, for example settling of components. This can result in uncontrolled, unidentified or unmonitored changes in a desired injection quantity or in a profile with respect to time of a combustion process. Conventionally, a determination of the stroke values or of the change in the stroke values during the service life of the injector has not been performed.